1. Field of the Invention
The present invention generally relates to non-invasive techniques for diagnosis of medical conditions, and in particular to use of such techniques for monitoring pressure within the cranium.
2. Background Description
Pressure increase in the brain, which is contained within the skull, is a serious medical condition that can be life threatening. The brain is surrounded by cerebrospinal fluid; the pressure of the fluid, known as intra cranial pressure (ICP), is carefully controlled by homeostatic mechanisms in the body. In pathological circumstances such as stroke or head injury, elevated ICP poses a clinical problem that must be carefully managed to prevent severe brain damage or even the death of the patient. Intra cranial pressure changes can be detected non invasively using acoustic stimulation and analysis. The brain and the skull are coupled resonant systems that will respond in a predictable fashion to pressure increases given the bio-boundary conditions. Changes in acoustic damping are correlated with changes in cerebral spinal fluid (CSF) or intra cranial pressure (ICP). Intra cranial pressure is currently monitored invasively, with monitoring devices implanted within the skull cavity or lumbar punctures.
There are two “windows” to the interior of the skull for “seeing” brain pressure: the ear and the eye. Intra cranial pressure (ICP) in the brain is DIRECTLY communicated to both the eye and ear. The eye is more convenient to non invasively monitor changes in ICP. Ear monitoring of changes in cerebral spinal fluid (CSF) pressure have been attempted but have not resulted in a feasible clinical device. Direct measures of skull vibration by using ultrasonic probes have also been attempted, but with limited success because it is technically complicated, and is not a promising clinical alternative. Eye pressure does correlate with CSF pressure and various approaches have been used since eye pressure assessment is a common ophthalmological procedure.
Two types of intra ocular pressure measurement have been reported, with varying correlations to intra cranial pressure (ICP). These include non-contact air tonometry, which measures intra ocular pressure. This technique has produced conflicting results and at best provides only a rough estimate of ICP. The other reported technique is ophthalmodynometry, which is an aplanatic technique. This technique applies pressure to the cornea and measures the intra ocular arterial pulse wave. Pressure is applied to the corneal surface until the intra ocular arterial pulse wave (produced by the ophthalmic artery) is obliterated. The pressure at which this happens has been termed intra cranial arterial pressure and some have used this pressure to infer changes in ICP. However, this measure cannot be equated with ICP.
There have been previous attempts by other researchers to acoustically measure ICP in animals, but although they have proven the validity of the concept, they have not been practical for portable use in humans. Semmlow and Fisher (“A noninvasive approach to Intra cranial pressure monitoring”, Journal of Clinical Engineering, 1982, vol. 7, pp. 73-78) observed in young dogs that the response of the head to low-level audible vibrations correlates with ICP elevation. Stevanovic et al. (“The effect of elevated Intra cranial pressure on the vibrational response of the bovine head”, Annals of Biomedical Engineering, 1995, vol. 23, pp. 720-727) demonstrated the correlation of audible vibration with ICP in sheep, artificially elevating ICP and monitoring the acoustic signal. However, although their method did not require surgery within the skull cavity, it did require implantation of screws in the skull, making it impractical for a portable system.
There is a need for a technique that can be used to non-invasively monitor intra cranial pressure (ICP). There is a need for a portable, non-invasive sensor for measuring ICP in casualties with traumatic head injury, suitable for use in the battle area and during medical evacuation as well as in hospital conditions. This will allow monitoring to prevent secondary brain damage. Timely identification and treatment of elevated ICP will greatly improve the chances of survival of these patients.